Despite the shrinking of Islamic State-held territory in Iraq and Syria, the head of the Central Intelligence Agency is stressing that the group’s influence is not going away anytime soon.

“I think ISIL [Islamic State] will remain a presence inside Iraq and Syria for quite a while,” CIA Director John Brennan said Thursday during a national security summit in Washington.

Brennan also highlighted the lasting impact of foreign fighters attempting to return to their home countries. "While some of them may be rehabilitated and see they were on the wrong path, I do think a number of them will remain a challenge for the United States as well as for other governments for years to come,” he said.

Intelligence officials acknowledge that dismantling IS now will most likely have wider repercussions and present broader challenges for law enforcement as a whole.

“Through the fingers of that crush are going to come hundreds of hardened killers who are not going to die on the battlefield, and they’re going to flow out," said FBI Director James Comey.

Being able to anticipate the possible flow of foreign fighters into Western Europe will require combined resources across agencies and countries, intelligence officials said. But that long-term strategy can be hard to execute.

“We’ve done fairly well in winning the battles, but have not always done well in winning the wars,” said Lieutenant General Vincent Stewart, director of the Defense Intelligence Agency.

The progression of intelligence sharing on a global level is also key to a successful counterterrorism strategy, a lesson learned through experience, summit participants said.

“There has been even a broader effort to bring in a number of other countries' intelligence services, because the [IS] phenomenon does affect a much broader swath of the globe than I think al-Qaida ever did,” Brennan said.

When asked about the challenges facing Syria in particular, Brennan said the country represented one of the most complicated issues he had faced during his tenure as CIA chief.

“Even though we’re able to reverse some of the battlefield successes of ISIL in Syria, I think the challenges will increase in terms of political reforms, economic reforms, social and religious tensions, and sectarian conflict that is taking place” he said.